When trying to click "Repair Disk" on an external hard drive from Disk Utility, I got the following error message:

...
Repairing the EFI system partition's file system
Updating boot support partitions for the volume as required.
Creating a new EFI system partition
Problems were encountered during repair of the partition map:
Error: Couldn't repair the partition map because a new EFI system partition couldn't be created.

What does this error message mean? Is it just complaining because there is no EFI system partition, or is it something more serious?

This is an 3TB external hard disk I use for external storage (e.g., Time Machine backups, other data), and it is not bootable. diskutil list shows that it is partitioned with 2 partitions (one for Time Machine, one for data) but no EFI partition. I suspect the partition table was created under Linux rather than Mac OS X. The disk has been working fine up until recently. I did read Why does Disk Utility create EFI partitions on external disks?; that and other sources gave me the impression that a EFI partition is not strictly required unless you want to boot from the disk.